1. Field of the Invention
The present invention relates to a focus information display system, and more particularly, to a focus information display system that displays focus information indicating a focus state of whether a correct focus state exists based on a focus evaluation value indicating a contrast intensity of a subject image formed by an optical system.
2. Description of the Related Art
Contrast method automatic focusing (AF) is generally known as an AF function that is adopted in broadcast television cameras. With contrast method AF, signals of high frequency components are extracted from visual image signals obtained via a camera, and a focus evaluation value that evaluates contrast intensity is obtained based on the extracted signals. The focusing of the photographing lens (focusing lens group) is controlled using, for instance, a method referred to as the hill-climbing method so that a peak (maximum) focus evaluation value is obtained.
Additionally, in contrast method AF, an operation referred to as wobbling is performed in which focusing is subjected to minute fluctuations. By performing focus evaluation value detection while wobbling is in progress, for instance, focus evaluation values at front and rear positions of a current focus position may be detected. Based on the magnitude relation of the focus evaluation values of the front and rear positions, detection is performed on whether the focus evaluation value at the current position is a peak value, or otherwise a direction in which the focus evaluation value will reach its peak value is detected. The focus is moved to a correct focus position at which a peak focus evaluation value is achieved by controlling focusing while sequentially detecting peak focus evaluation values caused by wobbling.
Furthermore, when performing wobbling as described above, there are problems in that focus fluctuation due to wobbling are recognized on screen, or accurate focusing on subjects moving at high-speed is difficult. In this light, an optical path length difference method AF, such as that presented in Japanese Patent Application Laid-Open No. 2004-212458, is proposed. According to Japanese Patent Application Laid-Open No. 2004-212458, an optical path of a photographing lens (optical system) that forms a subject image is split into two optical paths by a light splitting device such as a half mirror or the like. Subject light that is guided to one of the optical paths forms a subject image on an image capturing surface of an image capturing element (visual image capturing element) of a camera main body that is arranged to capture images (visual images) for purposes of recording or playback. Subject light that is guided to the other optical path forms a subject image on an image capturing surface of an image capturing element (AF image capturing element) that is arranged to capture AF images (visual images).
The image capturing surface of the AF image capturing element is comprised of, for instance, two image capturing surfaces with optical path lengths arranged at different positions using two image capturing elements. Subject light that is diverged to the AF optical path is further split to incident to respective image capturing surfaces of the AF image capturing elements. The image (subject image) captured by each image capturing surface of the AF image capturing element corresponds to an image captured by the image capturing surface of the visual image capturing element when focus is displaced from a current position by a given amount. A focus state (correct focus, front focus, rear focus) at the current position of the focus in relation to an image capturing surface of the visual image capturing element is therefore arranged to be detectable by obtaining a focus evaluation value from each image captured by the respective image capturing surfaces of the AF image capturing elements and comparing the obtained focus evaluation values.
Thus, since there is no need to perform wobbling, focus may be moved to a correct focus position without creating focus fluctuation attributable to wobbling. In addition, prompt focusing may be achieved, and accurate focusing may even be performed on subjects moving at high speeds.
Furthermore, an invention proposed in Japanese Patent Application Laid-Open No. 2002-365710 is arranged so that focus state detection performed under AF, as described above, is also performed during manual focusing (MF), and focus information indicating a current focus state is displayed on a view finder of a camera or a display unit of a focusing section or the like. According to Japanese Patent Application Laid-Open No. 2002-365710, by performing focusing using the focus information as reference, a photographer may easily determine, not only from images on the view finder, whether a subject is in focus or otherwise determine a focusing direction when not in focus.